Further studies in the acyl-type radical additions promoted by SmI 2 : mechanistic implications and stereoselective reduction of the keto-functionality
作者:Lise M Mikkelsen、Christina M Jensen、Bettina Høj、Peter Blakskjær、Troels Skrydstrup
DOI:10.1016/j.tet.2003.07.016
日期:2003.12
Attempts were made to promote the carbonyl coupling of cyclohexanone to 4-pyridylthioesters of N-carbamate-protected amino acids with the one electron reducing agent, samarium diiodide. Such reactions proved unsuccessful due to the inability of the ketyl-type radical anion intermediate to be reduced to the corresponding dianion at -78degreesC. Nevertheless, these results explain our recently published work on the high efficiency of the SmI2-mediated acyl-type radical additions of the same thioesters with electron deficient alkenes [J. Am. Chem. Soc. 2003, 125, 4030]. A study was also undertaken to examine methods for the stereoselective reduction of N-carbamate-protected amino ketones to either the syn- or anti-vicinal amino alcohols. In most cases, LiAl(0-t-Bu)(3)H and (S)-Alpine-Hydride were found to effectively provide the anti- and syn-amino alcohols, respectively. The SmI2-promoted reduction of the same ketones afforded a majority of the syn-isomer with selectivities of approximately 5:1. However, in one case, the SmI2-promoted reduction was found to be more effective than that of (S)-Alpine-Hydride. (C) 2003 Elsevier Ltd. All rights reserved.